1. Field of the Invention
This invention relates to electro-chemical cells and batteries and more particularly to zinc halogen cells and a construction of such cells and batteries is described in the United Kingdom Letters Pat. Specification No. 1,258,502.
2. Description of Prior Art
U.S. Pat. No. 1,258,502 claims an electrode structure for a rechargeable zinc halogen cell comprising a substrate of anodizable metal selected from the metals of Group IV(A) or Group V(A) of the Periodic Table according to Mendeleef having permanently associated therewith at least one coherent stratum of substantially porous carbon. The electrode is produced by pressing a mixture of the porous carbon and a particulate binder onto the substrate at a pressure of at least 2 tons p.s.i. A halogen storage electrode comprises at least two spaced electrode structures as described above, preferably in plate or like form, joined together along one, two or three edges, and a covering of synthetic resin material inert to the halogen secured in a gas-tight manner round the edges leaving the surface of the carbon stratum free and leaving one end of the electrode open to permit gas access to the enclosed space between the electrode structures, provision being made for an electrical connection to the metal substrate.
A suitable electrode for use in the rechargeable electric zinc halogen cell or battery comprises a substrate of the anodizable metal of Group IV(A) or V(A) of open mesh, apertured, expanded metal or porous metal form coated with zinc or a zinc salt or an alloy containing zinc, mercury, indium or gallium.
Moreover, our co-pending Application No. 40297/73 discloses improvements in such cells in which inter alia the halogen e.g. chlorine electrode is sealed to provide a pressure differential across the electrode. The halogen may however be bromine or iodine.
The rechargeable cell or battery of such cells may comprise a casing with titanium end walls and titanium walls separating electrode compartments in the casing, a number of chlorine storage carbon/metal electrodes mounted between two consecutive titanium walls interspaced with zinc bearing electrodes, and a halogen gas inlet to the casing and communicating with the open bottom portions of the carbon/metal electrodes which form a common channel for the gas in the cell, the carbon/metal electrodes and the zinc bearing electrodes being electrically connected respectively to positive and negative poles of the cell or battery. The cell or battery is in a halogen gas circuit including a halogen gas cylinder, a pressure regulator valve, an ultraviolet lamp for the removal of undesirable gases such as hydrogen from the circuit, and a halogen gas liquifier connected to the cylinder, and the lamp being in the circuit between the valve and the liquifier.
The carbon/metal electrode structure is preferably about 30 to 50% porous so that the halogen gas, preferably chlorine, stored in it acts as the cathode. The electrolyte is the chloride of the halogen used, preferably zinc chloride.
It has been found that during charging the halogen notably chlorine is evolved on the outside of the carbon/metal cathode and if this reaches the zinc bearing electrode the latter would be subject to a degree of corrosion, dendrites tend to form which shorten the life of the cell, and the pH value of the electrolyte is approximately the same throughout the cell whereas a pH difference at the cathode and anode e.g. 1.0 and 2.0 respectively is found to be advantageous.
The main object of the present invention is to provide a cell or battery of cells in which the aforesaid points can be employed at what are the present time believed to be optimum values.
It has been found that by employing a semi-permeable material in sheet form as a membrane between the cathode and the anode, the chlorine diffusion in the cell is reduced, the pH value of the electrolyte can be maintained at desirable and different values adjacent to the cathode and anode and dendrite formation is reduced: the plating efficiency of the zinc bearing anode is also improved.